This study is designed to identify the safety and maximally tolerated dose of aerosolized INS365 in adult and pediatric cystic fibrosis patients with mild to moderate lung disease. The safety and tolerability of four dose levels of INS365, administered as single inhaled doses via jet nebulizer will be investigated. P2Y2 receptor agonists may have beneficial effects in patients with cystic fibrosis lung disease by improving mucociliary clearance with less cough and bronchoconstriction than other drugs. The primary objectives of this are to: 1) evaluate the safety of escalating single doses of aerosolized INS365 in adult and pediatric cystic fibrosis patients with mild to moderate lung disease. 2) determine the maximally tolerated dose of aerosolized INS365 in adult and pediatric cystic fibrosis patients with mild to moderate lung disease. The secondary objective of this study is to: 1) obtain descriptive information on the effect of INS365 versus placebo on weight of sputum produced following a single dose of aerosolized INS365.